The use of discrete biscyclopentadienyl-based and monocyclopentadienyl-based metal complexes for the polymerization of olefins is well known in the art. In a few cases, olefin polymerization has been demonstrated starting from discrete catalyst precursor complexes with cyclopentadienyl-based ancillary ligand systems and reduced oxidation state metal centers such as in, for example, U.S. Pat. Nos. 5,374,696 and 5,494,874, both to Rosen, et. al.; WO 96/13529; and Theopold, Acc. Chem. Res., vol. 23, pp.263-270 (1990). However, these catalyst precursor complexes do not exhibit C.sub.2 or pseudo-C.sub.2 symmetries, useful symmetries with many metallocene catalysts.
Recently, there has been an increased interest in identifying catalytic systems that incorporate non-cyclopentadienyl ancillary ligands. For example, Canich and Turner, U.S. Pat. No. 5,318,935 discloses bisamido Group 4 transition metal compounds and McConville, et. al., Macromolecules, vol. 29, p. 5241 (1996), discloses bridged, dianionic, diamide ligands. These catalysts incorporate d.sup.0 metals in their highest oxidation states. Both of WO 96/23010 and Gibson, et. al., Chem. Comm., pp. 849-85 (1998), disclose diimine-based ligands for metals in Groups 8-10. These diimine-based ancillary ligands are neutral donor ligands. Other demonstrated examples of catalyst precursor complexes incorporating non-cyclopentadienyl ancillary ligands and reduced oxidation state metals show these compounds to have very low activity, see WO 97/17379.
Organometallic compounds with anionic, non-cyclopentadienyl ligands, including those with reduced oxidation state metal centers and those structurally characterized as having C.sub.2 or pseudo-C.sub.2 symmetry, are known in the scientific literature. See, for example, Fryzuk, Can. J. Chem., vol. 70, p. 2839 (1992); Edwards, et. al., J. Chem. Soc., Dalton Trans., p. 1253 (1989); and van Koten, et. al., J. Am. Chem. Soc., vol. 104, p. 5490 (1982). However, the teachings of these documents do not suggest potential utility of the compounds as polymerization catalysts or catalyst precursor compounds.